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I did run it on a single battery for many years, and various other people (such as but not limited to my old mate Tannin) used to start my car for me in the mornings 'coz my battery was always flat. Mind you, they were happy enough the night before drinking the cold beers out of the fridge that flattened the battery that wouldn't start the car, so I reckon the whole thing was fair enough

Thanks for the info guys Im still deciding but Im thinking the evakool is the way to go.

Really? How big is the fridge?

Fridge is 35 or 40 litres IIRC.

Size is not the issue: its power consumption.

According to the site, the average power consumption is Ave. power consumption: 0.85 amps/hr (@ 12 volts, 5 °C interior, 32 °C ambient temperature).

I round that out to 1 amp per hour. A 20 watt panel will put out 20 amps per hour at the same voltage. Am I gonna use 20 amps every hour? No, am I gonna use an amp for every hour of the day? Not that either.

The panel is more than adequate to compensate for the fridge's draw on the 100 amp spare battery.

You are definitely not correct about the power output of a 20W Panel. According to the specs from Jaycar the panel will put out a maximum of 1.3 amps and only in full sun and only for sunny hours of the day.

If you say there are 12 hours of full sun a day which is pretty conservative that is only 0.65 amps per hour on average output from the solar panel which is less than the fridge average usage.